A direct-acting engine, used as a blowing-engine, was not constructed until about 1784, at which time a single-acting blowing-cylinder, or air-pump, was placed at the "out-board" end of the beam, where the pump-rod had been attached. The piston of the air-cylinder was loaded with the weights needed to force it down, expelling the air, and the engine did its work in raising the loaded piston, the air-cylinder filling as the piston rose. A large "accumulator" was used to equalize the pressure of the expelled air. This consisted of another air-cylinder, having a loaded piston which was left free to rise and fall. At each expulsion of air by the blowing-engine this cylinder was filled, the loaded piston rising to the top. While the piston of the former was returning, and the air-cylinder was taking in its charge of air, the accumulator would gradually discharge the stored air, the piston slowly falling under its load. This piston was called the "floating piston," or "fly-piston," and its action was, in effect, precisely that of the upper portion of the common blacksmith's bellows. Dr. Robison, the author of "Mechanical Philosophy," one of the very few works even now existing deserving such a title, describes one of these engines[34] as working in Scotland in 1790. It had a steam-cylinder 40 or 44 inches in diameter, a blowing-cylinder 60 inches in diameter, and the stroke of piston was 6 feet. The air-pressure was 2.77 pounds per square inch as a maximum in the blowing-cylinder; and the floating piston in the regulating-cylinder was loaded with 2.63 pounds per square inch. Making 15 or 18 strokes per minute, this engine delivered about 1,600 cubic feet of air, or 120-1/2 pounds in weight, per minute, and developed 20 horse-power. [34] "Encyclopædia Britannica," 1st edition. At about the same date a change was made in the blowing-cylinder. The air entered at the bottom, as before, but was forced out at the top, the piston being fitted with valves, as in the common lifting-pump, and the engine thus being arranged to do the work of expulsion during the down-stroke of the steam-piston.